The present invention relates to a method for reading information or detecting defects of a disc rotating at a constant angular velocity.
Recently there has been developed various kinds of systems in which information is recorded on a disc along spiral or concentric tracks and is read out of the rotating disc by means of a pick-up head which is moved in a radial direction of the disc. For instance, a video disc system has been developed in which a video signal is converted into a digital signal and the converted digital signal is recorded on an optical disc as a pit array along spiral or concentrical tracks and the recorded signal is then read out by means of a scanning light beam projected from a pick-up head which is moved in the radial direction of the disc. Upon reproduction, the video disc is rotated at a constant angular velocity. In order to obtain a reproduced digital signal having the same pulse duration as that of the original digital signal, pits on the disc must have different lengths at different positions viewed in its radial direction, because a linear velocity is different at the different positions on the disc. That is to say, the pits on inner tracks have a shorter length than on outer tracks. It should be noted that such a system does not utilize a recording density of the disc in an optimum manner. That is to say, on the outer tracks, the signal is not recorded with the highest recording density which can be obtained in the innermost track. In order to avoid such a drawback, it has been also known to record the signal on the disc in such a manner that the pits have the same length on both the outer and inner tracks. For example, such a system has been applied to a digital audio disc. In this system, in order to reproduce the signal correctly, the disc is rotated at a constant linear velocity, i.e. at different angular velocities. Contrary to this system, if the disc is rotated at a constant angular velocity, durations of reproduced pulses corresponding to the pits become different from each other at different points on the disc viewed in its radial direction.
Now this will be explained further in detail with reference to FIGS. 1 and 2A to 2D. In FIG. 1, a disc 1 is rotated in a direction shown by an arrow 2 at a constant angular velocity and information pits 3, 4 and 5 having the same length are recorded on different tracks. It should be noted that the pits are shown exaggeratedly. When these pits 3, 4 and 5 are picked up at a signal detecting position 6 on the basis of a rotation signal shown in FIG. 2A, there are obtained pit signals having durations which are inversely proportional to distances from a rotating center of the disc 1 to respective pits as shown in FIGS. 2B-2D. Therefore, if the information is defined by the pit lengths on the disc, the picked up information signals shown in FIGS. 2B-2D are detected to express different information in spite of the fact that the pits have been recorded as having the same information. For instance, a pulse width modulation signal could not be reproduced correctly.
In order to eliminate the drawback mentioned above, there has been developed a method for rotating the disc at a uniform linear velocity as mentioned above. In this method, a command signal for denoting a rotation speed of the disc to attain the uniform linear velocity must be recorded along respective tracks on the disc, and upon the reproduction the command signal is at first read out by the pick-up head, and then a rotating speed of the disc is controlled according to the picked-up command signal so as to obtain a required linear velocity at respective positions on the disc. However, if the pick-up head is moved abruptly in the radial direction, information reading can not be initiated until the disc is rotated at the required linear velocity by means of a disc rotating mechanism having a large time lag or inertia. Therefore, an access time might be prolonged to a great extent. Moreover, if the command signal is not recorded on the disc, the control of the disc rotation could not be effected at all. For instance, in case of detecting defects on a disc in which information has not yet been recorded, the disc could not be rotated at a constant linear velocity. Further, when defects on silicon wafers for use in manufacturing semiconductor devices are to be detected by concentrically or spirally scanning the wafer surfaces, the wafers could not be rotated at a constant linear velocity, because the command signal could never be recorded thereon. In such a defect detection, it is important to detect size or dimension of defects precisely, but in the known methods utilizing the constant angular velocity, the size or dimension of defects could not be detected accurately.